An organic EL element is a self-luminous element, so that a brighter, clearer display is possible compared to a liquid crystal element. Further, it has useful characteristics such as a wide view angle and high-speed responsibility. Accordingly, studies thereon have been made by many researchers from long ago.
Initially, an organic electroluminescence element using an organic material had been far from a practical level. However, characteristics thereof have been dramatically improved by a laminated structure element developed by C. W. Tang et al of Eastman Kodak Co. in 1987, in which various roles are divided to respective materials. They laminated a fluorescent material which is stable in the structure of its vapor-deposited film and can transport electrons, with organic matter which can transport holes, and injected both carriers into the fluorescent material, thereby succeeding in emitting light. This improved the luminous efficiency of the organic electroluminescence element, resulting in obtaining a high luminance of 1000 cd/m2 or more at a voltage of 10 V or less (for example, see patent document 1 and patent document 2). Thereafter, studies for improving the characteristics were made by many researchers, and at present, the luminous characteristic of a higher luminance of 10000 cd/m2 or more have been obtained concerning light emission for a short period of time.
Patent Document 1: JP-A-8-48656
Patent Document 2: Japanese Patent No. 3194657
At present, the organic EL elements have been put to practical use, and utilized as displays for cell phones, car audios and the like, and it has been further expected to enlarge the size and to expand the use range. However, there are still many problems required to be solved. One of them is heat resistance at the time when they are driven under high temperature environment. α-NPD which has been widely used as a hole transport material at present has a problem in heat resistance, and the use thereof under high temperature environment such as a large-sized display which generates heat upon use or in-vehicle applications which require high durability has been considered to be impossible (for example, see non-patent document 1). Accordingly, concerning a presently employed general element constitution, it has been said that the heat stability of the element is determined by the heat stability of the hole transport material. This is because the hole transport material mainly composed of an organic amine-based material is inevitably disadvantageous in terms of heat stability, when attention is paid to the materials of the respective layers used in organic EL elements. Accordingly, improvements of the heat stability of the hole transport material is considered to lead to improvements of the heat stability of the element. The general element constitution referred to herein indicates one as shown in FIG. 1.
Non-Patent Document 1: M&BE, vol. 11, No. 1 (2000)